sandbox: allow processing before main loop

In order to pass command line arguments to sandbox we need to be able
to act on them. So take control back at the end of board_init_r() from
where we can call the main loop or do something else.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/arch/sandbox/include/asm/u-boot-sandbox.h b/arch/sandbox/include/asm/u-boot-sandbox.h
index 236b4ee..99e950b 100644
--- a/arch/sandbox/include/asm/u-boot-sandbox.h
+++ b/arch/sandbox/include/asm/u-boot-sandbox.h
@@ -35,4 +35,7 @@
 int board_init(void);
 int dram_init(void);
 
+/* start.c */
+int sandbox_main_loop_init(void);
+
 #endif	/* _U_BOOT_SANDBOX_H_ */